feat: tipos y funciones email SMTP en Go (infra)

Tipos: EmailAttachment, EmailMessage, SMTPConfig.
Puras: email_build_html, email_build_text, email_with_attachment, email_template_render.
Impuras: smtp_connect (TLS/STARTTLS/plain), smtp_send (MIME multipart con adjuntos).
Solo stdlib: net/smtp, crypto/tls, text/template, mime/multipart.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

functions/infra/email_template_render.go

@@ -0,0 +1,24 @@
+package infra
+
+import (
+	"bytes"
+	"fmt"
+	"text/template"
+)
+
+// EmailTemplateRender executes a Go text/template with the given data map
+// and returns the rendered string.
+// tmplStr is the template source (Go text/template syntax).
+// data is a map of string keys to any values available inside the template.
+// Returns an error if the template fails to parse or execute.
+func EmailTemplateRender(tmplStr string, data map[string]any) (string, error) {
+	tmpl, err := template.New("email").Parse(tmplStr)
+	if err != nil {
+		return "", fmt.Errorf("email_template_render: parse: %w", err)
+	}
+	var buf bytes.Buffer
+	if err := tmpl.Execute(&buf, data); err != nil {
+		return "", fmt.Errorf("email_template_render: execute: %w", err)
+	}
+	return buf.String(), nil
+}